Answer
$x=6$
Work Step by Step
Let
$\log_{4}{4^6} = x$
RECALL:
$\log_b{y} =x \longrightarrow b^x=y$
Use the rule above, where $y=4^6$ and $b=4$, to obtain:
$4^x=4^6$
Use the rule "If $a^x=a^y$, then $x=y$" to obtain:
$x=6$